1. Field of the Invention
The present invention relates to a printing apparatus and a printhead temperature retaining control method. Particularly, the present invention relates to a printing apparatus which causes a printhead to discharge ink to print an image on a printing medium, and a temperature retaining control method for the printhead.
2. Description of the Related Art
In recent years, the need for high-performance printing apparatuses used as a printer, copying machine, facsimile apparatus, and the like is growing, and a printing apparatus is required to be capable of not only high-speed printing or full-color printing but also high-resolution image printing with the quality of silver halide photos. Regarding these requests, inkjet printing apparatuses which can implement high-speed high-quality printing by discharging very small ink droplets at a high frequency are superior to printing apparatuses employing other printing methods. Among inkjet printing apparatuses, a printing apparatus employing a thermal inkjet printing method of discharging ink using bubbles generated by heaters (electrothermal transducers) can have nozzles at a high density and therefore print a high-quality image.
The above-described thermal inkjet printing method (to be simply referred to as an inkjet printing method hereinafter) has the following characteristic features.
In the inkjet printing method, electric power is supplied to heaters to generate thermal energy which forms bubbles in ink. Growth of the bubbles is largely influenced by ink temperature in the vicinity. At the interfaces between bubbles and ink, a process of popping molecules in the gaseous phase from the bubbles to the ink and a process of popping molecules in the liquid phase from the ink to the bubbles occur. The ink temperature near bubbles influences the latter process. If the ink temperature is high, many molecules pop out from the ink to the bubbles, and the bubbles grow relatively large. Conversely, if the ink temperature is low, the number of molecules popping out from the ink to the bubbles is relatively small, and the bubble size is smaller than that at a high ink temperature. The bubble size is reflected on the volume (to be referred to as an ink discharge amount hereinafter) of ink pushed out from the nozzles or the ink discharge speed (to be referred to as a discharge speed hereinafter).
Hence, in the inkjet printing apparatus, the amount of ink discharge and discharge speed are greatly affected by the ink temperature near the heaters (to be referred to as an ink temperature hereinafter). If the ink temperature is high, the amount of ink discharge is large, and the discharge speed is high. On the other hand, if the ink temperature is low, the amount of ink discharge is small, and the discharge speed is low.
As another characteristic feature of the inkjet printing method, the temperature near the heaters becomes higher during printing than before the start of printing.
This is because the thermal energy generated by the heaters does not totally contribute to bubble forming. A surplus obtained by subtracting the energy required for bubble forming from the thermal energy is stored as thermal energy in ambient ink and members such as a printhead substrate. The stored thermal energy is dissipated by thermal conduction or thermal radiation. However, since the heaters continue to supply thermal energy during printing, the ink temperature continues to rise when the amount of thermal energy dissipation is smaller than the amount of supply. On the other hand, during non-printing without thermal energy supply from the heaters, the ink temperature continues to drop down to the equilibrium state with the ambient temperature. In other words, portions that are printed at a high ink temperature and portions that are printed at a low ink temperature almost equal to room temperature exist on a printing medium depending on the number of times of heater drive, that is, print data.
For this reason, the amount of ink discharge changes between the high-temperature portions and the low-temperature portions. Particularly in printing a photo-quality image, the density of the output image or the ink landing positions on the printing medium change. This may cause density unevenness on the printed image and degrade the printing quality.
Conventionally, a temperature retaining control method is known as a measure against variations in the amount of ink discharge and discharge speed depending on the ink temperature. This method holds the printhead at a relatively high predetermined temperature, thereby suppressing variations in the amount of ink discharge and discharge speed. In a method proposed in, for example, Japanese Patent Laid-Open No. 6-278291, a temperature (reference temperature) capable of reducing the variation width of the amount of ink discharge is predetermined, and the printhead is heated up to the reference temperature during a non-printing state (non-printing period) in which preparations for printing such as printing medium conveyance and preliminary discharge are performed.
In methods proposed in Japanese Patent Laid-Open Nos. 8-336962, 11-192727, and 11-342604, the printhead is heated during a non-printing state including an acceleration period during printhead scanning for the above-described temperature retaining control.
The temperature retaining control is effective for the above-described problems of variations in the amount of ink discharge and discharge speed and also advantageous because ink discharge can be performed with lower thermal energy. This is because heated ink has a low viscosity and can flow easily. More specifically, the temperature retaining control enables ink discharge with lower thermal energy and also makes it possible to discharge more viscous ink while maintaining the thermal energy to be used, and therefore extend the range of choices of ink.
The conventional temperature retaining control methods have many advantages. Temperature retaining control starts in the non-printing state before the start of printing and continues until the start of printing and during printing (printing period). However, if heating is always continued to maintain the relatively high predetermined ink temperature from the non-printing state to the start of printing, the power consumption increases. Additionally, if temperature retaining control is always continued from the non-printing state to the start of printing and during printing, a large amount of heat is stored in the printhead. This may raise the printhead temperature more than necessary at the start of ink discharge immediately after the start of printing and make ink discharge unstable, resulting in degradation in image quality.
According to Japanese Patent Laid-Open No. 6-278291, a temperature (reference temperature) capable of reducing the variation width of the amount of ink discharge is predetermined. The printhead is heated up to the reference temperature during the non-printing state, and printing starts from this state. However, as described above, if heating is always continued to maintain the high ink temperature in the non-printing state to prepare for printing, the power consumption increases. Additionally, if the high ink temperature is maintained for a long time without ink discharge, the printhead stores a large amount of heat. This may raise the printhead temperature more than necessary at the start of ink discharge immediately after the start of printing and make ink discharge unstable, resulting in degradation in image quality.
In the temperature retaining control methods proposed in Japanese Patent Laid-Open Nos. 8-336962, 11-192727, and 11-342604, the printhead is heated during the non-printing state including an acceleration period during printhead scanning, instead of always heating the printhead from the non-printing state. These prior arts can solve the problem of power consumption of temperature retaining control in the non-printing state by limiting the temperature retaining control period in the non-printing state to prepare for printing. However, these methods continuously execute the temperature retaining control from its start to the start of printing. This may make ink discharge immediately after the start of printing unstable, resulting in degradation in image quality.
Furthermore, the conventional temperature retaining control aims at ink discharge during printing, and ink discharge executed in the non-printing state is not addressed. That is, no optimum temperature retaining control has been proposed for ink discharge (to be referred to as preliminary discharge hereinafter) which is periodically executed without using a printing medium in the non-printing state, that is, before the start of printing, during conveyance of a printing medium, or after the end of printing to prevent the ink from increasing the viscosity or solidifying when left stand for a long time or prevent mixture of ink colors.